A Driving Force

A mocap studio test-drives a 3D car 'stunt double'

By Karen Moltenbrey

Performance Capture Studios (PCS) in Culver City, California, prides itself on breaking pre-established limits when it comes to motion capture. In the past, the company has not only captured the movements of people and mapped them onto digital characters, but used the technology on unique actors from the animal kingdom, including 5000-pound elephants, a Siberian lynx, monkeys, kangaroos, spiders, and frogs. In a recent project, the "actor" whose motion PCS captured-a car-became the driving force for helping the company establish a new direction in motion capture.

"I believe this is the first complete car capture ever done. We knew it was possible, but just hadn't had a need for it-until now," says Gary Roberts, PCS president. "Most people equate motion capture with human movement for producing similar-looking characters, but we try to push the envelope and show that the technology can be used for driving unorthodox characters with vastly different body topologies to that of an actor."

The studio was approached by the producers of a feature film currently in preproduction (the name of which PCS cannot yet divulge) to test the feasibility of capturing the movement of a car on a live-action set. The client's goal was to map the car data onto a computer-generated vehicle that would replace a physical car in the scene. "They wanted this futuristic-looking car in the film without having to build a physical model, which is expensive," says Roberts. The producers also needed all the interactions of the car with the people and scenery on the set to be realistic and believable. More important, he adds, the live actors had to react and interact with the CG car in a totally realistic way, as the stuntmen crashed into and then rolled over the vehicle.

Performance Capture Studios captured the motion of a moving car and mapped it onto a stylized 3D model of the vehicle. On film, this resulted in realistic interaction between the actors and the CG car.

To accomplish that feat, PCS placed 70 markers on the vehicle to capture the entire physicality of the car for every shot in the test-from the doors and wheels to the fuel cap and suspension. "We placed the markers where we would get good contact points with the actors and scenery when we retargeted the motion onto the CG car," Roberts explains. "Even if the shot called for the car to nudge someone with the bumper, we captured the motion of the entire car, which gives the production team and art director a tremendous amount of options for putting the shot together." For the actual movie though, in some shots PCS may only capture specific portions of the car to make the postproduction easier.

Before the actual motion-capture session began, PCS went car shopping, visiting numerous dealerships in search of a production vehicle that fit the physical dimensions of the film's futuristic style of car. This was done so the various parts-tires, hood, trunk, windshield, wheelbase, etc.-occupied the same physical space as the CG version, provided by the client as an Alias| Wave front Maya model. After selecting a Chrysler LHS, the group was ready to hit the road.

Using a Motion Analysis Real Time Optical Capture system, PCS placed 32 cam eras within a 120- by 60-foot capture volume-one of the largest ever achieved for the entertainment industry. This enabled Roberts, who has professional driving experience, to perform some of the maneuvers. At times he reached 50 mph just before entering the capture volume, set up inside a soundstage where the action was shot. (Qualified stuntmen performed the more dangerous moves.) "With the mocap system, we were amazed at how accurate we were in the size volume, and how good the data looked," says Roberts.

While the team had conducted small tests with the capture volume in its studio, it didn't have the chance to test the setup on the live-action set prior to the preproduction trial because of the other setup and equipment required for the shoot.

By viewing a 3D representation of the car from different perspectives as the vehicle drove through the motion-capture volume, the client and director could see the captured action in real time during the shoot.

To map the motion data onto the CG car, the group used proprietary in-house software that's based on the specific biomechanical principles of the subject, whether it's a person, animal, or in this case, a vehicle. "Typically with a cartoon character, we build a biomechanical knowledge base of how the character is going to move, so we can accomplish a good retargeting solution," says Michele Barbera, executive producer for PCS. "With the car, we had to change some of the software code, so we were essentially building a full physical and dynamic model of the car, but with a simplified suspension system. Once we did that, we could retarget the motion from the Chrysler onto the 3D car and still retain every subtle nuance such as the tires compressing and rotating." This "motion retargeting" was done in real time during the shoot so the director could see the digital outline of the car performing the stunts as they were occurring on set.

One of the biggest challenges to accomplishing this moving experience was making sure that the set lights did not interfere with the cameras used in the shoot. "Our equipment works with strong visible red light, and we were able to control some of the lighting on the set by having the technicians remove as much red light from their equipment as possible," explains Roberts. The group also used a special lens filter for the PCS cameras that blocked out extraneous light rays. The team took the added measure of taping or painting the car's highly reflective surfaces.

Another obstacle resulted from the speed of the car-in particular, the tire rotation. To keep pace, PCS had to capture that motion at high speed (120 frames per second) so it could be used to accurately generate the motion of the 3D car. Once the data was captured and a physical simulation of the CG car was generated, the group sampled the data down to traditional film speed. They then generated a pipeline that enabled the client's animation team to apply the car capture data to the 3D scene and layer in keyframe animation where required.

Roberts concedes that motion capture isn't the solution for all character animation, but if it's used correctly, surprising results can be achieved. In other instances, animation and modeling software can produce the same effects as motion capture. For instance, in the case of the car capture, there is good physical simulation software that can model the dynamic forces of cars, and together with keyframing, an animator can generate fairly realistic car motion, Roberts adds. However, the main goal for the client in this instance was to generate the close interactions of live actors with the CG car on screen.

"It was interesting seeing the results of the real car being captured and interacting with live stuntmen, and seeing a totally convincing CG car interacting with the same live actors," says Patrick Miller, vice president of applications engineering at Motion Analysis.

Performance Capture Studios placed 70 markers on the vehicle so that every motion of the car could be captured-from the wheels to the windshield.

According to Barbera, motion tracking could have produced similar results in this instance. "The clients wanted complete freedom to change things in postproduction, and they wanted to accurately track the car in true 3D so many of the CG elements and cameras could be driven by the 3D path of the car," she says. "They were also filming the action from many viewpoints. So in this case, motion capture and our PCS technology allowed for a more automated and accurate approach-we tracked the motion of the car down to a fraction of a millimeter. It would also give us more creative freedom in post, with less manual effort."

Barbera and Roberts hope that the car capture will spawn further innovation in mocap, outside of human motion capture-what's been the comfort zone and norm for the industry thus far," says Barbera. "With the right technology, a creative mind, and the right team, motion capture can breathe life and soul into CG characters."